Minjie Zhu1, Aditya Jhajharia1, and Dirk Mayer1
1Diagnostic Radiology & Nuclear Medicine, University of Maryland Baltimore, Baltimore, MD, United States
Synopsis
Dynamic metabolic imaging was used to investigate the origin of the
lactate signal in rat brain after injection with hyperpolarized (HP) [1-13C] pyruvate.
In order to suppress signal with vascular components, we applied a bipolar gradient pulse
with different duration prior to data acquisition. The presented results provide further evidence that transport of HP
pyruvate into the brain and its subsequent conversion to lactate is limited by
blood-brain barrier transport and that the majority of the HP lactate observed
in brain was indeed produced there.
Introduction
Metabolic imaging of hyperpolarized
(HP) 13C-pyruvate has been applied in multiple rodent studies to investigate brain
metabolism in healthy animals1-5 and disease models such as brain
tumor6-11, traumatic brain injury12,13, and
neuroinflammation14. A recent report by Miller et al.15 concluded: “earlier reports of whole-brain metabolism in anesthetized animals may
be confounded by partial volume effects and not informative enough for translational
studies” after the authors did not observe HP 13C-lactate in the brain of
healthy rats or in normal-appearing brain tissue of animals with brain tumors.
The goal of this study was to further investigate the origin of the lactate
signal in rat brain after injection with HP [1-13C]pyruvate. To this end, we
used a dynamic spiral chemical shift imaging(spCSI) sequence16 that was
modified to include a bipolar gradient pulse prior to data acquisition to
suppress signal components from flowing spins17.Methods
All measurements were performed on a
clinical 3T GE 750w MR scanner (GE Healthcare, Waukesha, WI, USA) using a 1H-13C dual-tuned
RF coil
(i.d. = 50 mm) for both excitation and signal reception. The gradient system
had a maximum amplitude of 33 mT/m and a slew rate of 120 T/m/s. In vivo
experiments using spCSI sequence without and with different bipolar gradient
widths W (6ms and 9.6ms for a single lobe, 30 mT/m along z and 23.3 mT/m
along x and y) were performed on two healthy male Wistar rats
(265-270g body weight). Animals were anesthetized with isoflurane (1.7-2% in
1L/min O2) throughout the imaging session. Each rat received a
1µmol/g dose of 125mM hyperpolarized pyruvate (approximately 2.2 mL) injected
over ~6 seconds through a tail vein catheter, followed by 0.5mL saline flush.
Scan parameters for the 2D dynamic spCSI sequence were: 40mm FOV, 2.5x2.5mm2
nominal in-plane resolution, 10mm slice thickness in axial orientation (animal
coronal) centered in the middle of the brain, 8 spatial interleaves, 280Hz
spectral width with 24 echoes, 3s temporal sampling. The echo time (TE) was
adjusted for the different suppression levels: TE=5.6ms for W=0, TE=16ms for
W=6ms, and TE=23.2ms for W=9.6ms. A constant 5.625° flip angle excitation with
16 time points over 48 seconds (starting at 6s after the start of pyruvate
injection) were used for the first rat. A variable flip angle scheme, $$$ \theta_i =
\arctan{\frac{1}{\sqrt{48 - i}}} $$$, was used for the 48 excitations for 6 time points over
18 seconds (starting at 9s after the start of pyruvate injection) for the second
rat. The imaging order was: W = 6ms, W = 0, and W = 9.6ms for both animals. Results and Discussion
Time-averaged metabolic maps of
pyruvate and lactate from three acquisitions with different suppression levels
from the two animals are shown in Figure 1. For each map only the data
corresponding to the 19 echoes starting at TE=23.2ms was used achieve the same T2*
weighting. Three regions of interest (ROIs) were selected to quantify the
effect of flow suppression – brain, vasculature, and tongue/muscle tissue.
Figure 2 shows the mean pyruvate and lactate intensity in the brain and
vascular ROIs normalized to the mean lactate of tongue/muscle ROI for each
acquisition to correct for different polarization levels. The signal reduction
relative to the acquisition without flow suppression for pyruvate and lactate
in the selected ROIs averaged for the two animals are shown in Figure 3. As
expected, the strongest signal reduction was observed in the vascular ROI for
both pyruvate and lactate that increased with duration of the suppression
pulse. Pyruvate in the brain ROI showed similar behavior. This is consistent
with transport across the blood-brain barrier (BBB) being the rate-limiting
step in the production of HP lactate in brain and the pyruvate observed in the
brain ROI being predominately in the cerebral blood volume1. In
contrast, lactate in the brain ROI showed the least amount of reduction, only
about 20% for both suppression levels. Altogether these results suggest that the brain lactate signal is predominantly stationary or slower moving. Dynamic 13C
data of rat 1 acquired with 6 ms bipolar gradient width are shown in Figures 4
and 5. Although the transport of lactate across the BBB is on the order of 3 to
10 times faster compared to pyruvate18 the early appearance of
lactate in the brain ROI strongly supports that the lactate signal in rat brain
was most likely due to metabolic conversion of pyruvate in the brain, rather
than lactate being produced in other organs, released into the circulatory
system, and eventually taken up by the brain.Conclusion
The presented results
provide further evidence that transport of HP pyruvate into the brain and its subsequent
conversion to lactate is limited by BBB transport. The data further supports
that the majority of the HP lactate observed in the brain was indeed produced there
rather than due to partial volume effects or uptake of lactate elsewhere in the
body. Therefore, metabolic imaging of HP pyruvate could be a useful tool for
the investigation of brain metabolism in rodent studies.Acknowledgements
This work was supported by NIH grants R21 NS096575, R01 DK106395, R21
CA213020, and R21 CA202694. References
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